1. Field of the Invention
The present invention relates to the field of electron paramagnetic resonance (EPR) instruments and, more particularly, to EPR instruments sufficiently sensitive to perform rapid, real time EPR measurements on large, lossy specimens.
2. Description of the Prior Art
Those concerned with measuring radiation damage to objects have, in many cases, successfully employed EPR techniques. Although prior art EPR devices have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reason that considerable difficulty has been experienced in making rapid, real time EPR measurements on relatively large, lossy specimens.
For example, an urgent military need has long existed for instruments capable of making accurate, real-time determinations of radiation damage to a soldier having been exposed to gamma and/or neutron radiation. It is well known that such radiation breaks bonds between atoms in molecules. When the bonds are homolytically broken, free radicals result. The free radicals can be observed in an EPR spectrometer. Human components, such as the finger or tooth, are recognized as being potentially excellent samples to utilize for determining radiation damage to an individual. Free radicals in the bones resonate when exposed to microwaves at a frequency of about 9.5 GHZ (X-band). However, the inside dimensions of an X-bank waveguide are only 0.4 by 0.9 inches for 8.2 to 12.4 GHZ. An X-bank rectangular cavity has the same cross-section dimensions and a length equal to an integral number of 1/2 guide wavelengths. It can be seen, therefore, that a finger would occupy a significant portion of an EPR X-bank cavity. As such, the size of a lossy sample is usually limited to only a few microliters for EPR measurements because larger, lossy samples would drastically reduce the Q of the cavity so that EPR measurements are impossible.
U.S. Pat. No. 4,674,513, entitled "Active Microwave Cavity for Electron Paramagnetic Resonance (EPR) Apparatus" discusses in detail many technical barriers that must be overcome before a large lossy item, such as a finger, can be utilized as an EPR sample. The apparatus disclosed in the '513 patent is directed to an EPR instrument wherein a microwave amplifier has an active X-bank cavity which can be utilized for making EPR measurements on a large size sample (&lt;50 milliliters). The RF energy is transported into the cavity by an electron beam carrying an amplified RF wave. The RF wave is essentially "trapped" in the cavity because of the non-reciprocal nature of the electron beam medium and because the guide at the input to the cavity has dimensions such that the frequency of the RF microwave energy is below the guide's lower cut-off frequency. The "trapped" RF energy in the cavity generates large, narrow resonant peaks in the cavity and a cavity Q-factor greater than 1000 can be realized. Although the EPR instrument of the '513 patent solves the problem of making X-bank EPR measurements on large, lossy samples, it does so at the expense of using a substantial amount of additional power. Most of the additional power is needed to operate the electron beam apparatus. Also, the additional construction costs of the microwave amplifier and the active microwave cavity may be substantial due to the need for the generation and focusing of an electron beam in a vacuum.
The problems associated with designing EPR cavities are also discussed by Robert Allendoerfer and James Carroll, Jr. in an article entitled "A Coaxial Microwave Cavity for improved Electron Paramagnetic Resonance Sensitivity with Lossy Solvents", Journal of Magnetic Resonance, 37, pp. 497-508 (1980), incorporated herein by reference. In this article, the authors state on page 503 that the Q of a typical X-band TE011 cylindrical cavity is about 10,000 without a sample. When the optimum amount of the sample is added, the resulting loaded Q is about 1700, and this value is approaching the minimum value that spectrometers can use for EPR measurements. It is also noted that the optimum sample thickness is in the micron range which is much less than the thickness of a finger.
Therefore, those concerned with the development of EPR measurement apparatus have long recognized the need for a more sensitive device that can make accurate real-time measurements on large, lossy samples. The present invention fulfills this need.